Why it is needed to invoke parent constructor?

Question

public class AAA
public AAA(int a) {  }
public class BBB extends AAA
public BBB(int a) { super(a); }
public BBB() {  }
It gives an error in the second constructor of class BBB. Why? Why Java requires AAA to have an AAA() constructor???
Maciek

Answer 1

public class AAA
public AAA(int a) {  }
public class BBB extends AAA
public BBB(int a) { super(a); }
public BBB() {  }
It gives an error in the second constructor of class
BBB. Why? Because you have not explicitely called a parent constructor, the compiler will add an implicit call to the no-args parent constructor thus:
super();But the no-args constructor does not exist in the superclass!
The compiler only creates a default constructor if you do not explicitely declare any constructors, so since you have declared a constructor that takes an argument, and have not declared a constructor that takes no arguments, AAA does not have a no-args constructor.
Why Java requires AAA to have an AAA()
constructor???It doesn't - it only requires it to have at least one constructor. But the code you have in BBB tries to call the non-existent AAA() constructor. You must either explicitely declare the no-args constructor, or explicitely call an existing constructor from every constructor in every subclass.